Water pump replacement

My water pump on an '81 Excella II has died on me and I need to replace it. The existing pump is a Shurflo 45psi one which seems to be dated 2007, so it was a replacement itself. As the less powerful models from Shurflo cost less and presumably do the job O.K. can anyone tell me which psi model it originally would have had - perhaps 30psi? The less powerful models cost less and there's no sense paying for more than I need. The pump is only for pumping the water to kitchen/bathroom and doesn't feed any radiators or anything else. Any advice much appreciated - Geoff B.